Arrangement for preventing the formation of cracks on the inside surfaces of feedwater line nozzles opening into pressure vessels

ABSTRACT

Arrangement for the prevention of crack formations at the inside surfaces of feedwater line nozzles (13) which open into pressure vessels, especially nuclear reactor pressure vessels or steam generators. The feedwater is introduced into the water/steam space (II) of the pressure vessel (DE) via a substantially horizontal line section (140) and a rising line section (141) following downstream thereof up to the overflow edges U at the end of the flow travel of the rising line section (141). From there, the feedwater is admixed to the medium in the water/steam space (II) or the descent space (8) of the pressure vessel (DE) via a downward-directed line section (141) and optionally, via a ring feedline (12) connected thereto. This prevents temperature stratification due to backflow of warmer water in the nozzle. It is important here to make the ratio A n  /D i  as small as possible. In practice it has been found to be feasible to provide a ratio which is approximately within the limits 0.5 and 2. (A n ) here refers to the horizontal distance of the pressure vessel inside wall from the center line (M u ) going through the center of gravity of that cross-sectional surface which is defined by the overflow edges (U). (D i ) refers to the inside diameter of the feedwater line opening into the pressure vessel.

The invention relates to an arrangement for preventing the formation ofcracks at the inside surfaces of feedwater line nozzles which open intopressure vessels, particularly nuclear reactor pressure vessels or steamgenerators.

Such an arrangement is known for steam generators of nuclear reactorsfrom German Published Non-Prosecuted Application 23 46 411.

If a cold medium is replenished to a system (vessel, pipeline) which isfilled with a medium of elevated temperature, via a horizontalconnection, i.e. a feedwater line nozzle, a stratification of colder andwarmer medium comes about in the horizontal connecting piece, if thefeeding takes place with a mass throughput which is small relative tothe size of the connecting cross section. i.e. with low flow velocities.The same thing occurs if the medium is already present in the system tobe fed in evaporated condition. The stratification comes about becausethe lighter (warmer) medium can flow back into the upper part of theflow cross section of the nozzle due to its buoyancy against the colderwater to be fed-in. The temperature differences of the media lead tothermal stresses in the connecting nozzle or the connecting line whichas a rule are already highly stressed by the internal pressure of thesystem, so that with a sufficiently large number of cycles of thefeeding processes, material fatigue and thereby, the formation of crackscan occur. The phenomenon of temperature stratification could bedemonstrated by temperature measurements on feedwater line nozzles of asteam generator for pressurized-water reactors.

In the known arrangement according to the German PublishedNon-Prosecuted Application mentioned at the outset, the feeding takesplace with a flow conducted in the horizontal and/or downward-directeddirection within the vessel. Although the feed nozzles are lined attheir inner periphery with so-called thermosleeve tubes, the thermalstress problems mentioned at the outset can nevertheless still occur insubregions of the nozzles.

It is an object of the invention to develop the arrangement of the typementioned at the outset in such a manner that for slow feeding of thefeedwater line nozzle such as occurs under partial load or zero loadoperation of a plant, material fatigue and thereby, the crack formationat the nozzle are impossible with certainty, even with larger numbers ofload changes of the feeding processes. According to the invention, thestated problem is solved by the feature that the feedwater is fed intothe water/steam space of the pressure vessel through a substantiallyhorizontal line section and a following rising line section up to theoverflow edge at the end of the flow travel of the rising line section,from where the feedwater is admixed through a downwardly-directed linesection, and optionally through a ring feedline connected thereto, withthe medium in the water/steam space and in the descent space of thepressure vessel, so that temperature stratification due to flowback ofwarmer water in the nozzle is prevented. An advantageous furtherembodiment claim is that the line section which is substantiallyhorizontal, is followed first, through a bend, by a substantiallydownwardly-directed line portion which leads into a collecting cup thathas risisng flow paths up to the overflow edges. The advantagesattainable with the invention are in particular that a reverse flow ofthe specifically lighter medium in the fed system against the heavier(still cold) medium still to be fed-in is prevented. Through theinvention, not only all the problems of thermal stresses and crackformation which occur particularly under zero load and low loadoperation solved, but also those occurring during starting-up andshut-down operation.

In the following, the invention will be explained in greater detail withthe aid of the drawing showing several embodiment examples, and theoperation described. In schematic, simplified presentation, omitting theparts not required for an understanding of the invention,

FIG. 1 is a longitudinal section of a steam generator forpressurized-water reactors with a feedwater line nozzle designedaccording to the invention;

FIG. 2 is a fragmentary, enlarged view of the detail X from FIG. 1,being simplified;

FIG. 3 is a fragmentary cross section according to line III-III fromFIG. 2;

FIG. 4 is a presentation corresponding to FIG. 2 of, another embodimentof the arrangement which is constructed particularly low in thedirection of the nozzle axis;

FIG. 5 is a fragmentary cross section according to the line V--V of FIG.4;

FIG. 6 is a fragmentary cross section of a further embodiment whereinpart of the pressure vessel wall is also drawn, with particularly largeflow cross section;

FIG. 7 is a fragmentary cross section according to line VII--VII of FIG.6, and

FIG. 8 is a fragmentary cross section of a fourth version with adownward leading feedline section and collecting cup; and

FIG. 9 is a table of the variables A_(n) and D_(i).

The steam generator DE for pressurized-water reactors according to FIG.1 (called DE for short in the following) has a pressure vessel housing 1with a primary chamber region 1.1, an evaporator region 1.2 which hasthe U-shaped heat-exchanger tubes 2, and a separating region 1.4following via a conically expanding housing transition region 1.3. Thetube sheet 3 welded into the housing 1 and the heat exchanger tubes 2welded into it and held by it, gastightly separate the primary chamber Ifrom the secondary chamber II. The primary chamber I is formed by aspherical bottom section 4 with inflow nozzle E and outflow nozzle Awelded into the tube sheet 3, the inflow space el of the primary chamberbeing separated from the outflow space al by a dished partition 5. Ofthe tubes 2 of the tube bundle 2' only the outer and inner ones areindicated by lines; the tube bends are designated with reference numeral2.1 and the innertube lane with 2.2. The primary medium (water) which isheated in the core of the pressurized-water reactor, not shown, is fedat a temperature of about 316° C. and with a pressure of 155 bar to theprimary chamber I via the inlet nozzle E, flows through heat-exchangingtubes 2 and is fed back into the reactor pressure vessel via the outletchamber a1 and the outlet nozzle A with a temperature of about 290° C.

The tube bundle of the heat-exchanging tubes 2 is held vibrationproof bymeans of tube holding grids 6 axially spaced from each other, it issurrounded by a hollow cylindrical jacket 7 which forms, together withthe wall 1, an annular descent space 8. Since the jacket 7 is disposedat a distance a2 from the tube sheet 3, the descent space 8 is flow-wisein communication at its lower end with the evaporation space in theinterior of the jacket 7 via the flow lanes 8.1. At its upper end, thejacket 7 is terminated by an extension 9 which carries at its top abattery of water separators 10 into which the water/steam mixture entersthrough suitable flow canals from the evaporation space II. The ejectedwater, (the water level of the circulating water is indicated atreference numeral 11) is fed back directly into the descent space 8. Thering line 12, which is arranged at the upper end of the descent space,serves, via openings not shown, for feeding the feedwater from afeedwater line nozzle 13 via an essentially vertical connecting line 14.

The largely dehydrated steam which leaves the water separators 10 attheir top, then further flows into the steam purifier 15 and from there,via the live-steam nozzle 16 of the steam dome 17 to the steam turbines,not shown.

The steam generator works according to the natural circulationprinciple. The feedwater and the separated water flow mixed downward inthe descent space 8 and into the evaporation space II and rise in thelatter while being evaporated (wet steam). The water/steam mixture isthen transported into the coarse separators 10 and finally into thesteam purifiers 15 as already indicated. For introducing the feedwatervia the nozzle 13 and the connecting pipe 14, entirely defined flowmanagement is provided which will be explained with the aid of FIGS. 2and 3.

The feedwater is introduced into the water/steam space of the steamgenerator via a substantially horizontal line section 140 and a linesection 141 which follows downstream thereof and is designed as a pipebend up to the overflow edges U at the end of the flow travel end of therising section of the line 141. From there, the feedwater (see flowarrows f1) is admixed to the water/steam space via a downward-directedline section 142 and the feed ringline 12 connected thereto (FIG. 1),i.e. in this case to the descent space 8 of the steam generator. Theline section 142 is of domelike shape and comprises the line portion 141as a kind of bell. The line section 140 may be held in the nozzle 13 inthe manner of a thermo sleeve tube (FIG. 1). Because of the linearrangement described, backflow of already heated feedwater into theline section 140 is no longer possible. Such a load state is also alwayspresent because the colder inflowing feedwater must fill the nozzlecross section completely due to its higher specific gravity before itreaches the higher-positioned overload edges U.

The low compact design according to FIGS. 4 and 5 is recommended forsteam generators or reactor pressure vessels, in which only a littlespace is available in the direction of the nozzle axis. Like parts carrythe same reference symbols. Here the rising line part is formed by atray 141' of rectangular cross section which is flat and boxlike, andtherefore forms a waterbox, at the two narrow top edges on which theoverflow edges U are arranged. This water box 141' is surrounded by alikewise approximately boxlike structure 142 which is rounded at its topedge, for the downward-directed line section which may likewise be bentcorresponding to the inside circumference curvature of the pressurevessel, and which leads at its lower end into the ring line 12 via aconstricted neck piece 143.

The arrangement according to FIGS. 6 and 7 is intended for a stilllarger feedwater throughput at full load. The overflow edges U are herenot only the upper side edges 141.1, but also the edge 141.2 on thelongitudinal side of the line section 141. Accordingly, the flow crosssection of the downward-directed line section 142 is larger than in thataccording to FIGS. 4 and 5. It can furthermore be seen that the insidecircumference of the feedwater line nozzle 13, which is welded into thehousing wall of the steam generator by means of a circular welded seam18, is lined with the line part 140 which is designed as a thermo sleevepipe and is substantially horizontal. This thermo sleeve also may be acustomary design or be designed in accordance with FIGS. 2 of GermanPublished Non-Prosecuted Application 23 46 411.

In FIG. 8 it is further shown that a substantially horizontal linesection 140 can be followed, after an elbow 144, first by asubstantially downward-directed line section 145, which ends in acollecting cup 141" which has the ascending flow paths up to theoverflow edges U.

As tests have shown, it is important to maintain a certain ratio A_(n)/D_(i) in order to prevent the temperature stratification due toflowback of warmer water. In this context A_(n) refers to the horizontaldistance of the pressure vessel inside wall 1from the center line M_(u)which goes through the center of gravity of the cross section are F_(u)defined by the overflow edges U. D_(i) means the inside diameter of thefeedwater line 140 opening into the pressure vessel DE. Theabovementioned ratio A_(n) /D_(i) should be made as small as possibleand is for this purpose approximately within the limits of 0.5 and 2.

In FIGS. 2 and 3, the pressure vessel inside wall 1is shown in dashesand the center line M_(u) is drawn in a dot-dash line further, thedimension lines for the distance A_(n) =A_(l) and the inside diameterD_(i) are shown. From the table according to FIGS. 9, one obtains forthe embodiment example according to FIGS. 2 and 3; A_(l) =4.70, D_(i)=3.70 and accordingly A_(n) /D_(i) =1.27.

Still more favorable values for the ratio A_(n) /D_(i) are obtained forthe embodiment examples according to FIGS. 4, 5 and FIGS. 6, 7. For acomparative consideration of FIGS. 4 and the table according to FIG. 9,a value A₂ =2.15 is obtained, a value D_(i) =3.55 and accordingly, aratio A_(n) /D_(i) =0.61. This favorable value results from the compactlow design of the pipe line parts 141', 142. Correspondingly favorablevalues are obtained for the embodiment example according to FIGS. 6 and7 with A₃ =1.50, D_(i) =2.50 and A_(n) /D_(i) =0.60.

The embodiment example according to FIG. 8 shows in conjunction withFIG. 9 that there, the ratio A_(n) /D_(i) moves in the range of theupper limit of FIG. 2. As an advantage of this example, the largepassage cross section and the cylinder-symmetrical form should bementioned, which is also present, by the way, in the embodiment exampleaccording to FIGS. 2 and 3. In general it can be said thatcylinder-symmetrical shapes allow higher pressure stresses; box-shapedcross sections on the other hand have less pressure strength for a givenwall thickness, but the dimension in the direction of A_(n) is smaller.As a particularly advantageous design can therefore be considered theexample according to FIGS. 1 to 3, where a relatively low ratio A_(n)/D_(i) of 1.27 is realized and nevertheless, a relatively high pressurestrength is provided with sufficient flow cross section. Comparedtherewith, the other embodiment examples can be considered as specialdesigns in which either the ratio A_(n) /D_(i) is held particularly low(FIGS. 4 to 7) or the flow cross section in the overflow region isparticularly large (FIG. 8).

Overall, short flow paths to the overflow edges U can be arranged inrelation to the line cross section, so that the feedwater has noopportunity to heat up appreciably on the path to the overflow edges.Heating up is counteracted by the mass flow of the feedwater which ischaracterized by the quantity D_(i) which appears in the denominator ofthe ratio. The invention makes it possible in a relatively simple mannerto keep the path and thereby, the dwelling time of the feedwater, with amass throughput which is small relative to the flow cross section on thepath of the feedwater to the overflow edges, so small that detrimentalheating and thereby the consequent phenomenon of temperaturestratification and circulating flow up to the feedwater nozzle areprevented.

In the table according to FIG. 9, the centimeter values are made thebasis for A_(n) and D_(i) as they can be taken from anapproximately-scale drawing. In order to obtain a concept of feasiblescale dimensions, one should know that the pressure vessel of the steamgenerator according to FIG. 1 has an outside diameter of approximately4800 mm in its separator region 1.4 (steam dome), so that for thequantities A_(l) and D_(i) shown in the enlargement according to FIGS. 2and 3, values of approximately 500 mm and 400 mm, respectively, areobtained therefrom. The situation is then similar for the naturalmagnitudes of the A_(n) and D_(i) values of the other figures. The steamgenerator shown in FIG. 1 serves, for instance, together with threeother steam generators in a 4-loop arrangement, for generating theoperating steam in a 1200 MW_(el) pressurized-water nuclear powerstation.

We claim:
 1. Apparatus for preventing the formation of cracks in theinner surface of feedwater line nozzles which open into pressurevessels, the pressure vessels having a water/steam space and a descentspace for medium formed therein, comprising a substantially horizontalline section connected to a pressure vessel for feeding feedwater intothe water/steam space formed in the pressure vessel, a rising linesection having a given flow travel length and being disposed downstreamof said substantially horizontal line section, an overflow edge disposedat the end of said given flow travel length of said rising line section,and a downwardly-directed line section disposed at least partiallybetween said overflow edge and the descent space formed in the pressurevessel for admixing feedwater with the medium in the water/steam spaceand in the descent space for preventing temperature stratification dueto flowback to warmer water in the nozzle, wherein the ratio A_(n)/D_(i) is substantially between 0.5 and 2.0, where A_(n) is the distancealong the horizontal from the inner surface of the wall of the pressurevessel to a centerline passing through the center of gravity of thecross-sectional area defined by said overflow edge; and D_(i) is theinside diameter of said substantially horizontal line section connectedto the pressure vessel.
 2. Apparatus according to claim 1, wherein saidratio is between 0.6 and 1.5.
 3. Apparatus according to claim 1,including a ring feed line connected to said downwardly-directed linesection for conducting admixed feedwater and medium to said water/steamspace and descent space.
 4. Apparatus according to claim 1, wherein saidrising line section includes a horizontal portion, a bend connecteddownstream of said horizontal portion, a substantiallydownwardly-directed portion connected downstream of said bend and acollecting cup disposed downstream of said substantiallydownwardly-directed portion defining rising flow paths, said overflowedge being integral with said collecting cup at an end of said flowpaths.